1. Field of the Invention
This invention relates generally to circuit fabrication and, more particularly, to resistor fabrication.
2. Background of the Related Art
For a variety of reasons, a person's heart may not function properly and, thus, endanger the person's well-being. Medical devices have been developed to facilitate heart function. For instance, if a person's heart does not beat properly, a cardiac stimulator may be used to provide relief. A cardiac stimulator is a medical device that delivers electrical stimulation to a patient's heart. A cardiac stimulator generally includes a pulse generator for creating electrical stimulation pulses and a conductive lead for delivering these electrical stimulation pulses to the designated portion of the heart.
Cardiac stimulators generally fall into two categories, pacemakers and defibrillators, although some cardiac stimulators may perform both functions. Pacemakers supply electrical pulses to the heart to keep the heart beating at a desired rate. Defibrillators supply a relatively large electrical pulse to the heart to help the heart recover from cardiac failure.
Like many other electrical devices, the electrical circuitry used in cardiac stimulators tends to benefit from miniaturization. Although there are many reasons that miniaturization is beneficial, it should be understood that current cardiac stimulators are devices that are typically implanted into a patient's body. As is well known, smaller implanted devices are generally more desirable. Also, the functionality of cardiac stimulators has increased dramatically over the past few decades. Thus, improved miniaturization allows functionality to be added without increasing the overall size of the cardiac stimulator.
Although a variety of electrical elements may be subject to miniaturization, this disclosure concentrates on the fabrication of resistors. Resistors may be fabricated on a substrate, such as ceramic or a printed circuit board, by depositing a film of a resistive material, such as ruthenium, on the surface of the substrate. The resistors are typically deposited using a thick film process, such as screen printing. Because most thick film processes exhibit fairly high tolerance levels, it is quite normal for resistors deposited in this nature to exhibit resistance values that vary from the desired value. Therefore, such resistors are typically trimmed by measuring the resistance of the resistor and by removing a portion of the resistive material to alter the resistance. Typically, lasers are used in accordance with well known methods to trim resistors to the proper resistance level.
However, the creation of resistors tends to occupy a relatively large area on the surface of the substrate. Because resistors occupy such a large surface area in proportion to other circuit components which must be surface mounted, it would be desirable to form such resistors somewhere other than the surface. Unfortunately, because the entire circuit must typically be complete before the deposited resistive material may be fired to form the final resistor, a resistor cannot be trimmed to the appropriate resistance level unless it is deposited at the surface.
The present invention may address one or more of the problems set forth above.